1. Field of the Invention
The present invention relates to a positive active material of a lithium-sulfur battery and a method of fabricating the same, and more specifically, to a positive active material of a lithium-sulfur battery having high capacity and a method of fabricating the same.
2. Description of the Related Art
The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in high-tech electronic industries. Studies on secondary batteries are actively being pursued in accordance with the increased need for a battery having high energy density to use as a power source in portable electronic instruments. The secondary batteries are also required to satisfy demands of low cost, safety, and environmental affinity.
Lithium-sulfur batteries are expected to satisfy demands since active materials used in the batteries, lithium and sulfur, are cheap and environmentally friendly, as well as having high theoretical capacities. For example, the theoretical capacity of lithium, used as a negative active material, is about 3800 mAh/g, while the theoretical capacity of sulfur, used as a positive active material, is about 1650 mAh/g.
The lithium-sulfur batteries are secondary batteries composed of a positive electrode containing a sulfur-based compound having sulfur-sulfur bonds and a negative electrode such as an alkaline metal, e.g., lithium. The batteries produce and store electric energy as the results of a redox reaction in which the oxidation number of sulfur is decreased and sulfur-sulfur bonds are cleaved upon the reduction reaction (discharge), and the oxidation number of sulfur is increased and sulfur-sulfur bonds are formed upon the oxidation reaction (charge). The sulfur based materials including sulfur and polysulfides are referred to “active sulfur.”
The sulfur has complicated redox reactions where the polysulfides with various chain lengths are generated (E. Levillain et al., J. Electroanal. Chem. 420(1997) 167, J. Electroanal. Chem. 440(1997) 243). The net reaction occurring in the battery using active sulfur is illustrated by the following equation 1:xLi+S<--- >LixS  (1)
The theoretical final oxidation state is S8, and the final reduction state is Li2S. When S8 is completely reduced to Li2S (200% utilization in this patent), the theoretical capacity is estimated as 1675 mAh/g, which is a higher energy density than with any other chemicals.
The positive electrode is designed in either a manner in which active sulfur is dissolved in electrolyte (one phase positive electrode), or one in which the active sulfur coexists in partially solid (partial precipitation) and in partially solution phases. Refer to U.S. Pat. No. 6,030,720. Nevertheless, when either design is adopted, the active sulfur exists in both a solution phase and in a mixed phase of solid and liquid during progression of the charge and discharge in the lithium-sulfur battery (refer to FIG. 1). In FIG. 1, the solution phase is represented in a slope showing the sharp decline of discharge potential with the lapse of time. The plateau shows the region of coexisting solution and solid phases.
As a result, the positive active material of lithium-sulfur batteries exists in both solid and liquid phases, while that of other kind of batteries such as lithium ion batteries exists only as a solid. Lithium-sulfur batteries have a positive active material which may be attached to the positive electrode or dissolved in electrolyte.
Therefore, the positive active material may leak from the positive electrode into the separator or out of an electrode plate group (positive electrode, negative electrode, and separator) since it can be present in a flowing liquid phase. This renders the positive active material electrochemically inactive (incapable of participating in a redox reaction). Because this is an irreversible reaction, the lithium-sulfur battery does not display capabilities as well as is anticipated from the high theoretical capacity. Another problem is that the sulfur is precipitated on the surface of a conductive network by repeatedly reducing lithium polysulfide. This phenomenon seems to depend on the composition of the electrolyte.
To prevent these problems and to provide a lithium-sulfur battery having excellent capabilities, an organosulfur compound (U.S. Pat. Nos. 4,833,048, 4,917,974, and 5,162,175), DMcT-PAn (2,5-dimercapto-1,3,4-thiadiazol-polyanyline) (Oyama et al., Nature, 373, 598–600, 1995), and a carbon-sulfur compound (U.S. Pat. Nos. 5,441,831, 5,460,905, 5,601,947, and 5,609,720) have been suggested as a positive active material of lithium-sulfur batteries. However, these alternatives may cause problems, such as the organosulfur compound having a low theoretical capacity and a slow reaction rate, and the carbon-sulfur compound having a low theoretical capacity and not facilitating repeated realization of a material having the same molecular structure.
The elemental sulfur (S8) is expected to provide a positive electrode having high capacity and active material densities due to use of a powder form, which has the highest theoretical capacity. Elemental sulfur generally has no electrical conductivity, i.e., sulfur is a non-conductive material. Accordingly, to have an electrochemical reaction take place in the battery, an electrically conductive agent capable of providing a fluent electrochemical reaction must be added. U.S. Pat. Nos. 5,523,179 and 5,582,623 disclose a method to prepare an active sulfur-containing electrode comprising mixing sulfur and a conductive agent of carbon powder with a positive active material layer (mass). However, the sulfur is converted to polysulfide upon repeated charge and discharge cycles, then the polysulfide is effused to the electrolyte as a liquid phase in the aforementioned structure. Thus, the electrode structure collapses, decreasing the capacity and cycle-life characteristics of the lithium-sulfur battery.
In order to solve the above-cited problems, delaying the effluent of the positive active material by adding an absorbent capable of absorbing sulfur into a positive active material slurry is being studied. For the absorbent, Japanese un-Examined Patent Publication No. H09-147868 (Jun. 6, 1997) discloses an active carbon fiber, and U.S. Pat. No. 5,919,587 discloses a positive active material embedded between transition metal chalcogenides having a highly porous, fibrous, and ultra-fine sponge-like structure and a positive active material encapsulated with the same. WO 99/33131 discloses a method to add a particulate such as carbon, silica, or aluminum oxide having a potent absorbing characteristic to polysulfide. WO 99/33125 discloses a method to prevent the diffusion of the soluble polysulfide by encapsulating the positive electrode with a separator of a micro-porous pseudo-boehmite layer. WO 99/33127 discloses a cationic polymer comprising a quaternary ammonium salt group to make polysulfide anions stay around the cationic polymer. Nonetheless, the above disclosures do not avoid a deteriorating energy density due to an additional additive having specific functions.